Well i have a really annoying problem,

I have a small network with 5 computers sharing an access 2003 program.
when the database file become large in size , more than 15000 invoices, the
speed of my network drops down only if 2 or more users are signed in to the
program, but in case of 1 user only everything is working fine.

I pinged every client from the server and the result was normal (<1ms) .

My network is formed from 2 d-link switches, 4 clients are in one building
and the server is located in the next building with a distance not exceding
50 meters.

What could be wrong with such a network?

Any help will be Appreciated.
Thx.